Anyone for a South Pacific vacation after your Alaska Highway trip?  You can view some South Pacific geology in west-central Idaho on your way home!

As we drove north down the Little Clearwater River from our pine-forested lunch stop near Pinehurst, Idaho, we came around a curve and the vegetation changed completely.  The change was so abrupt that my wife commented that it looked like “it must rain a lot less here.”  But how could that happen in such a short distance?

The answer was that it couldn’t, so, having read about the area before our trip, I responded that it wasn’t the precipitation that changed, but the geology.  When we came around that curve, the pine-forested slopes of granite and basalt that we had been driving through became barely vegetated cliffs of metamorphic rock.  We had gone from the granite of the Idaho Batholith and Columbia Plateau Basalts into the Seven Devils complex.  To put it in terms discussed in the June 2 post, we had entered one of those “exotic terranes” where an island arc crashed into the North American Plate.

Geologists have dated the “crash” at about 90-100 million years ago (the Cretaceous Period, when some of the last dinosaurs, like the duck-bill dinosaurs and T. Rex, were walking around).  However, the rocks of the island arc were as much as 250 million years old (Permian and Triassic-age) and were formed in the warm waters of the South Pacific. The very large, tough, granitic Idaho Batholith was the western edge of the NA Plate, so the “softer” sedimentary rocks of the island arc simply smashed into an “immovable object” and were partly subducted and partly crushed (metamorphosed) into a thick sequence of gneiss and schist.  The partially subducted rocks are called the Seven Devils Complex and the highly metamorphosed rocks are called the Riggins Complex.

The following photo shows the Seven Devils Complex dipping (sloping) down toward the east (as if to dive under the Idaho Batholith granites of the NA Plate).  You can easily see how little vegetation there is in this exotic "terrane,” a sharp contrast to the conifer forest of the area just to the south along US 95.

The photo below shows the southwestward-dipping rocks of the Riggins Complex along the west bank of the Salmon River, just downstream from the whitewater-rafting mecca of Riggins, ID.  The Riggins Complex rocks apparently hit the edge of the Idaho Batholith and were bent upward, while the rocks of the Seven Devils Complex were forced under the Riggins rocks.  Notice the dip-controlled surface topography above the cliff (the sun-baked, smooth, grassy slope at the upper left).  That is Columbia Plateau Basalt overlying the Riggins Complex.  And why is the basalt dipping southwest too?  It is due to “isostasy,” the removal of the overlying weight of the rocks by the erosion by the Salmon River.  This “unloading” results in the underlying fluid mantle pushing the overlying continental crust up as the load is reduced (see my May 31 post about marshmallows and hot cocoa).  The same thing is still happening in the Midwest and Canada due to the removal of the ice load in the 14,000 years since the Wisconsin glaciation ended.

 

Below is a photo of a blocky, fractured outcrop of metamorphic rocks of the Riggins Complex north of the town of Riggins.  It would be very difficult to tell whether these mica schists were originally sedimentary or igneous rocks when they were deposited in the South Pacific Ocean 250 million years ago.

The photo below is a close-up of the same outcrop of Riggins Complex schists.  Note the way the sunshine on the rocks gives them a “sheen” caused by the planar alignment of the flat mica sheets that make up the schist.

After climbing a very long hill on US 95 northwest of White Bird, ID, we encountered another abrupt change from the rocks of the island arc back into the Columbia Plateau Basalts.  It will be interesting to see if the island arc terranes of the Okanagan Valley of central Washington and southern British Columbia have as abrupt contacts with the surrounding rocks as those of Idaho. 

And now, if you happen to be driving home from Alaska via US 95 from Coeur d’Alene to Boise, you will know exactly when you leave North America and enter the “South Pacific”!

Next time: Glaciation in the Coeur d’Alene to Grand Coulee region has immensely modified the Columbia Plateau basalt surface in east-central Washington State.